1. Field of the Invention
The present invention relates to a lockup device for a fluid-type torque transmission device. More specifically, the present invention relates to a lockup device provided in a fluid-type torque transmission device equipped with a front cover having a friction surface, an impeller that is fixed to the front cover and forms a fluid chamber, and a turbine arranged opposite the impeller inside the fluid chamber.
2. Background Information
A conventional torque converter has three types of bladed wheels (an impeller, a turbine, and a stator) arranged therein. A torque converter represents one type of fluid-type torque transmission device because it transmits torque through fluid enclosed therein. A torque converter is often provided with a lockup device. The lockup device is usually disposed in the space between the turbine and a front cover, which form a fluid chamber of the torque converter. The lockup device is a mechanism that serves to couple mechanically the front cover and the turbine together such that torque can be transmitted directly from the front cover to the turbine. Normally, the lockup device has a circular disc shaped piston, a drive plate, a driven plate, and a torsion spring. The piston can be pressed against the front cover. The drive plate is fixed to an outer circumferential part of the piston. The driven plate is fixed to the turbine. The torsion spring serves to couple elastically the drive plate and the driven plate together in the rotational direction.
When the lockup device is engaged, torque is transmitted from the front cover to the piston and from the piston to the turbine through the torsion spring. The torsion spring is compressed in the rotational direction between the drive plate and the driven plate and acts to absorb and damp torsional vibrations.
There have already been proposals for such a lockup device that has a plurality of friction surfaces to increase the torque transmission capacity. One such device has a drive plate, a driven plate, a plurality of torsion springs, and a piston. The drive plate is provided with a friction coupling part. The driven plate is joined together with the turbine. The plurality of torsion springs elastically couple the drive plate and driven plate together in the rotational direction. The piston presses the friction coupling part against the front cover. Here, the driven plate holds the outside circumference and rotationally facing ends of the torsion springs. The drive plate has an abutting part for abutting against the rotationally facing end parts of the torsion springs so that it can compress the torsion springs in the rotational direction. The friction coupling part of the drive plate is disposed axially between the piston and the front cover.
In this lockup device, the drive plate is an annular plate having the friction coupling part and abutting part formed integrally thereon. The abutting part of the drive plate is arranged such that it can abut against the torsion springs in the vicinity of the mounting radius of the torsion springs. Meanwhile, the friction coupling part of the drive plate must be arranged at a position that is more inward in the radial direction than the position of the abutting part because the friction coupling part is sandwiched axially between the piston and the front cover. Consequently, the friction coupling part cannot be positioned on the radially outside portion of the drive plate. Thus, the torque transmission capacity of the lockup device cannot be increased.
When such a lockup device is in the engaged state, torque is transmitted from the front cover to the drive plate and from the drive plate to the turbine through the torsion springs. In this state, the torsion springs are compressed in the rotational direction of the torque converter between the drive plate and the driven plate and act to absorb and damp torsional vibrations. The torsion springs also move radially outward due to centrifugal force and slide against the driven plate, which holds the torsion springs. Consequently, wearing of the torsion springs and the driven plate becomes a problem.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved lockup device for a fluid-type torque transmission device that overcomes the aforementioned problems. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.